[spark] Expose external table type to Spark

[huangxiaopingRD]

Purpose

Fix an inconsistency where Spark did not recognize Paimon external tables as external in Spark-facing metadata.
Before this change, a Paimon table could already be stored as table.type=EXTERNAL, but Spark-side logic relies on the reserved property TableCatalog.PROP_EXTERNAL to classify a table as external. Because that property was not exposed from PaimonSparkTableBase, Spark metadata output could report the wrong table type, such as showing a Paimon external table as managed in DESC FORMATTED.
This change fixes that by exposing PROP_EXTERNAL from PaimonSparkTableBase when the underlying Paimon table is external. That keeps Spark behavior consistent with the existing property-based contract already used by Spark-side commands like SHOW TABLE EXTENDED.

Tests

Adds a focused Hive catalog test to verify that:
DESC FORMATTED shows EXTERNAL for external tables
DESC FORMATTED shows MANAGED for managed tables